In rotary drilling there are a variety of functions and characteristics that are expected of a drilling fluid (“drilling mud” or simply “mud”). The drilling fluid is expected to carry cuttings from beneath the bit, transport them up the annulus, and permit their separation at the surface while at the same time the rotary bit is cooled and cleaned. A drilling mud is also intended to reduce friction between the drill string and the sides of the hole while maintaining the stability of uncased sections of the borehole. Likewise the drilling fluid is formulated to prevent unwanted influxes of formation fluids from permeable rocks penetrated and likewise to form a thin, low permeability filter cake, which seals pores and other openings and formations, penetrated by the bit. Finally, the drilling fluid is used to collect and interpret information available from drill cuttings, cores and electrical logs.
Drilling fluids are typically classified by the U.S. Environmental Protection Agency (“EPA”) in the following manner:
“Drilling Fluid” means the circulating fluid (mud) used in the rotary drilling of wells to clean and condition the hole and to counterbalance formation pressure. Classes of drilling fluids are:
(a) “Water-Based Drilling Fluid” means the continuous phase and suspending medium for solids is a water-miscible fluid, regardless of the presence of oil.
(b) “Non-Aqueous Drilling Fluid” means the continuous phase and suspending medium for solids is a water-immiscible fluid, such as oleaginous materials (e.g., mineral oil, enhanced mineral oil, paraffinic oil, C16–C18 internal olefins, and C8–C16 fatty acid/2-ethylhexyl esters).
(i) “Oil-Based” means the continuous phase of the drilling fluid consists of diesel oil, mineral oil, or some other oil, but contains no synthetic material or enhanced mineral oil.
(ii) “Enhanced Mineral Oil-Based” means the continuous phase of the drilling fluid is enhanced mineral oil.
(iii) “Synthetic-Based” means the continuous phase of the drilling fluid is a synthetic material or a combination of synthetic materials.
The EPA further defines Diesel oil, Enhanced Mineral Oil and Synthetic Material in the following manner:
“Diesel Oil” refers to the grade of distillate fuel oil, as specified in the American Society for Testing and Materials Standard Specification for Diesel Fuel Oils D975-91, that is typically used as the continuous phase in conventional oil-based drilling fluids. This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR Part 51. Copies may be obtained from the American Society for Testing and Materials, 1916 Race Street, Philadelphia, Pa. 19103. Copies may be inspected at the Office of the Federal Register, 800 North Capitol Street, NW., Suite 700, Washington, D.C. A copy may also be inspected at EPA's Water Docket, 401 M Street SW., Washington, D.C. 20460.
“Enhanced Mineral” oil as applied to enhanced mineral oil-based drilling fluid means a petroleum distillate which has been highly purified and is distinguished from diesel oil and conventional mineral oil in having a lower polycyclic aromatic hydrocarbon (PAH) content. Typically, conventional mineral oils have a PAH content on the order of 0.35 weight percent expressed as phenanthrene, whereas enhanced mineral oils typically have a PAH content of 0.001 or lower weight percent PAH expressed as phenanthrene.
“Synthetic Material” as applied to synthetic-based drilling fluid means material produced by the reaction of specific purified chemical feedstock, as opposed to the traditional base fluids such as diesel and mineral oil which are derived from crude oil solely through physical separation processes. Physical separation processes include fractionation and distillation and/or minor chemical reactions such as cracking and hydro processing. Since they are synthesized by the reaction of purified compounds, synthetic materials suitable for use in drilling fluids are typically free of polycyclic aromatic hydrocarbons (PAH's) but are sometimes found to contain levels of PAH up to 0.001 weight percent PAH expressed as phenanthrene. Internal olefins and vegetable esters are two examples of synthetic materials suitable for use by the oil and gas extraction industry in formulating drilling fluids. Internal olefins are synthesized from the isomerization of purified straight-chain (linear) hydrocarbons such as C16–C18 linear alpha olefins. C16–C18 linear alpha olefins are unsaturated hydrocarbons with the carbon to carbon double bond in the terminal position. Internal olefins are typically formed from heating linear alpha olefins with a catalyst. The feed material for synthetic linear alpha olefins is typically purified ethylene. Vegetable esters are synthesized from the acid-catalyzed esterification of vegetable fatty acids with various alcohols. EPA listed these two branches of synthetic fluid base materials to provide examples, and EPA does not mean to exclude other synthetic materials that are either in current use or may be used in the future. A synthetic-based drilling fluid may include a combination of synthetic materials.
On both offshore and inland drilling barges and rigs, drill cuttings are conveyed up the hole by a drilling fluid. Water based drilling fluids may be suitable for drilling in certain types of formations; however, for proper drilling in other formations, it is desirable to use a non-aqueous drilling fluid. With a non-aqueous drilling fluid, the cuttings, besides ordinarily containing moisture, are necessarily coated with an adherent film or layer of oleaginous drilling fluid, which may penetrate into the interior of the cuttings. This is true despite the use of various vibrating screens, mechanical separation devices and various chemical and washing techniques. Currently, in outer continental shelf waters of the U.S., cuttings drilled using diesel based and mineral oil based non-aqueous drilling fluids are prohibited from discharge and therefore require an alternate method of disposal in order to meet regulatory requirements, most of which are more expensive and more inconvenient than discharge of water based drilling fluids.
Regulations are imposed by governmental regulatory agencies concerning discharges of drilling fluids and associated cuttings. These regulations have not only made the use of diesel oil based and mineral oil based non-aqueous drilling fluid more costly but in some places difficult or impossible to use in compliance with the EPA guidelines and permits. Environmental concerns have prompted the development of a new drilling fluid. This drilling fluid is designed to meet the technology-based standards and water quality standards for discharge in compliance with EPA guidelines and Permits.
The EPA has specified protocol for meeting technology-based standards associated with discharge of cuttings associated with non-aqueous drilling fluids. These requirements can be found in the Effluent Limitation Guidelines for discharge of synthetic-based drilling mud and other non-aqueous based drilling fluids and the General permits that allow authorize discharge of synthetic-based drilling muds and other non-aqueous based drilling fluids. Detailed accounts of the procedures for measuring toxicity, biodegradation and PAH content and synthetic materials is described in the guidelines and in the permits. From the guidelines the following references are used:
Sediment toxicity: “Standard Guide for Conducting 10-day Static Sediment Toxicity Tests with Marine and Estuarine Amphipods,” 1992, after preparing the sediment according to the method specified in Appendix 3 of subpart A of 40 CFR part 435. This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR part 51. Copies may be obtained from the American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, Pa., 19428. Copies may be inspected at the Office of the Federal Register, 800 North Capitol Street, NW., Suite 700, Washington, D.C. A copy may also be inspected at EPA's Water Docket, 401 M Street SW., Washington, D.C. 20460.
Biodegradation: ISO 11734:1995 (specified at § 435.11(e)) method: “Water quality—Evaluation of the ‘ultimate’ anaerobic biodegradability of organic compounds in digested sludge—Method by measurement of the biogas production (1995 edition)” as modified for the marine environment (Appendix 4 of subpart A of 40 CFR part 435). This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR part 51. Copies may be obtained from the American National Standards Institute, 11 West 42nd Street, 13th Floor, New York, N.Y. 10036. Copies may be inspected at the Office of the Federal Register, 800 North Capitol Street, NW., Suite 700, Washington, D.C. A copy may also be inspected at EPA's Water Docket, 401 M Street SW., Washington, D.C. 20460.
PAH: Mass (g) of PAH (as phenanthrene)/Mass (g) of stock base fluid as determined by EPA Method 1654, Revision A, (specified at § 435.11(u)) entitled “PAH Content of Oil by HPLC/UV,” December 1992, which is published in Methods for the Determination of Diesel, Mineral, and Crude Oils in Offshore Oil and Gas Industry Discharges, EPA-821-R-92-008. This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR part 51. Copies may be obtained from the National Technical Information Service, Springfield, Va. 22161, 703-605-6000. Copies may be inspected at the Office of the Federal Register, 800 North Capitol Street, NW., Suite 700, Washington, D.C. A copy may also be inspected at EPA's Water Docket, 401 M Street SW., Washington, D.C. 20460.
Also described in the guidelines the procedures for determination or synthetic-based drilling mud drilling fluid toxicity is as follows:
4-day LC50 of C16–C18 internal olefin drilling fluid/4-day LC50 of drilling fluid removed from drill cuttings at the solids control equipment as determined by ASTM E 1367-92 (specified at § 435.11(ee)) method: “Standard Guide for Conducting 10-day Static Sediment Toxicity Tests with Marine and Estuarine Amphipods,” 1992, after preparing the sediment according to the method specified in Appendix 3 of subpart A of this part. This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR part 51. Copies may be obtained from the American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, Pa., 19428. Copies may be inspected at the Office of the Federal Register, 800 North Capitol Street, NW., Suite 700, Washington, D.C. A copy may also be inspected at EPA's Water Docket, 401 M Street SW., Washington, D.C. 20460.
These same procedures are further described in the EPA Region VI general permit GMG 290,000 as follows: Toxicity: Part I, Section D.8 of GMG 290000; Biodegradation: Part I, Section D.9 of GMG 290000; PAH: Part I, Section D.10 of GMG 290000
Several publications describe the use of mineral oils for low toxicity oil-based muds. However, mineral oils that were once considered to be toxicologically and environmentally superior to crude oil or diesel oil based drilling muds are now also prohibited from discharge under increasingly stringent environmental regulations. Several attempts to develop modified environmentally acceptable fluids have been made. In many instances these fluids are not true hydrocarbon fluids and require an aqueous continuous phase that does not provide desirable functional characteristics, for instance, shale stability derived with diesel oil based muds. Thus, there is a long felt need in the oil and gas exploration industry for an environmentally acceptable drilling fluid which either performs as well a diesel oil based drilling fluid has now been achieved by the present invention.